1. Technical Field
The invention relates to a sliding element, in particular a piston ring, having at least one sliding surface that has a coating which comprises, from the inside outwards, a metal-containing bonding layer and a DLC layer consisting of a covering layer and a main layer.
2. Related Art
Increasingly stricter legal provisions and heightened customer wishes for a lower fuel consumption by internal combustion engines require a sustained reduction in mechanical friction losses. However, this is also accompanied by significantly increased component requirements owing to the introduction of performance optimisations such as direct injection and turbocharging, which for their part require particularly wear-resistant components.
To safeguard the service life of highly stressed piston rings, hard-metal coatings are being increasingly used, such as described in DE 44 19 713. Metal nitrides, such as, for example, CrN and TiN, are thereby configured as a single or multilayer, mostly by means of physical vapour deposition.
In order to reduce the friction in the system piston ring/cylinder barrel face, amorphous DLC (diamond-like carbon) coatings are used (VDI Standard 2840). However, these have a service life that is still too short. And, owing to the formation of internal stresses that reduce the adhesiveness and thus the service life, the thickness of the layers to be applied is limited to approximately 5 him, such as described in DE 10 2005 063 123 B.
DE 40 40 456 C1 and DE 198 50 218 C1 describe the production of amorphous, hydrogen-free, diamond-like carbon layer systems using conventional sputtering processes or so-called vacuum arc processes. The high degree of hardness of the layers promises improved wear values. The hardness is thereby substantially determined by the bonding character, i.e. the ratio of sp2 to sp3 carbon bonds. The higher the sp3 content, the harder and more rigid the DLC layer.
The production of so-called hydrogen-free or oxygen- and hydrogen-free, amorphous, diamond-like layers is described in DE 10 2008 022 039 A1 and EP 0 724 023 A1.
However, the high degree of hardness and the high elastic modulus of amorphous, hydrogen-free, diamond-like carbon layers are also accompanied by some technological problems. On the one hand, the surfaces of the very hard layers must be very smooth in order to prevent the occurrence of disruptions on the surface at a high surface pressure and thus the failure of the layer system. Furthermore, a so-called bi-metallic effect occurs owing to different thermal expansion coefficients of the basic material of the piston ring and the coating thereof. This bi-metallic effect is intensified further owing to differences in the elastic modulus of the two materials. As a consequence, when the temperature increases, an increased pressure occurs in the joint region of the piston ring, which can cause increased wear. Furthermore, the partner element (cylinder barrel face) can be damaged by scoring and high liner wear.